Chemistry · Ch 5 — Coordination Chemistry
Coordination Compounds and Double Salts
Coordination Compounds and Double Salts
When two or more stable compounds in solution are mixed together and allowed to evaporate/crystallise, the result is sometimes a double salt and sometimes a coordination compound, and the two look deceptively similar on paper. The operational test that tells them apart is what happens in solution: a double salt loses its identity completely and dissociates into its simple constituent ions, exactly as if the starting salts had simply been mixed, whereas the complex ion of a coordination compound holds onto its identity and does not dissociate into those simple ions. Mohr's salt, ferrous ammonium sulphate FeSO₄.(NH₄)₂SO₄.6H₂O, made by crystallising equimolar ferrous sulphate and ammonium sulphate solutions, is the textbook double salt: testing it answers positive for free Fe²⁺, NH₄⁺ and SO₄²⁻ ions, because in solution it is really nothing more than a mixture of its two parent salts. Potassium ferrithiocyanate, K₃[Fe(SCN)₆], formed from the classic blood-red qualitative-analysis reaction between ferric chloride and potassium thiocyanate, is the contrasting coordination com …
Worked out. Mixing equimolar ferrous sulphate and ammonium sulphate solutions and crystallising gives Mohr's salt, FeSO₄.(NH₄)₂SO₄.6H₂O -- a double salt. A qualitative test on it answers positive for all three constituent simple ions, Fe²⁺, NH₄⁺ and SO₄²⁻, showing that a double salt loses its identity completely and dissociates into its simple ions in solution, exactly as if the two original salts had simply been mixed. Contrast this with the blood-red coordination compound formed when ferric chloride solution is treated with potassium thiocyanate solution: potassium ferrithiocyanate, K₃[Fe(SCN)₆]. A qualitative test on this compound does NOT answer positive for free Fe³⁺ or free SCN⁻ ions -- the complex ion [Fe(SCN)₆]³⁻ holds onto its identity and does not dissociate into simple ions the way a double salt does. This single experimental con …